RFR: 8286867: Update #getGlyphCode return a negative number

[Tomator]

On Tue, 17 May 2022 13:34:47 GMT, yosbits <duke at openjdk.java.net> wrote:

> Reading the author's description of this PR, one wonders why the added condition is not "glyphCode < 0".
> 
> ```java
> if (glyphCode <= 0) {return null;}
> ```

Your suggestion is right

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PR: https://git.openjdk.java.net/jfx/pull/795